In conventional or “wet” lithographic printing, ink-receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink-receptive regions accept the ink and repel the water. The ink is then transferred to the surface of a material upon which the image is to be reproduced. Typically, in a method known as “offset”, this is done indirectly by first transferring the ink to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
A class of imageable elements called printing plate precursors, useful for preparing lithographic printing plates, comprises a layer applied over the surface of a hydrophilic substrate. The layer includes one or more radiation-sensitive components, which may be dispersed in a suitable binder. Alternatively, or in addition, the binder itself may be radiation-sensitive. The layer is commonly applied as a coating, using a solvent.
If after exposure to radiation the exposed regions of the coating are removed in the developing process, revealing the underlying hydrophilic surface of the substrate, the plate precursor is referred to as “positive-working”. Conversely, if the unexposed regions are removed by the developing process and the exposed regions remain, the plate precursor is called “negative-working”.
In both cases, the regions of the radiation-sensitive layer (i.e., the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water, typically a fountain solution, and repel ink.
An alternative way of achieving the same result is to begin with a hydrophilic surface upon which, after imagewise exposure and developing, an ink-receptive pattern representing the image is obtained. If the unexposed areas become ink receptive, the plate precursor is “positive-working”, while if the exposed areas become ink receptive, it is “negative-working”.
Recent developments in the field of printing plate precursors deal with radiation-sensitive compositions that can be imagewise exposed by lasers or laser diodes. Because lasers can be controlled by computers, this type of imaging, known as digital imaging, does not require films as intermediate information carriers.
High-performance lasers or laser diodes typically used in commercially available exposure devices (known as platesetters) emit in the wavelength ranges of either 800 to 850 nm or 1060 to 1120 nm. Therefore, printing plate precursors, or initiator systems contained therein, which are to be imaged by such platesetters, must be sensitive to the near infrared. They are not, however, typically very sensitive to visible light. Such printing plate precursors can therefore basically be handled under daylight conditions, which significantly facilitates their production and processing.
Thermally imageable elements useful as lithographic printing plate precursors, exposable by infrared lasers or laser diodes as described above, are becoming increasingly important in the printing industry. After imagewise thermal exposure, the rate of removal of the exposed regions by a developer in positive-working elements is greater than the rate of removal of the unexposed regions, so that the exposed regions are removed by the developer to form an image.
Printing plate precursors are also in use which are imageable by ultraviolet radiation, as are types that are imageable by visible radiation.
Imaging of digital, thermally imageable precursors is typically done using platesetters, where the plate precursor is mounted either
i). on a rotatable drum (external drum), typically using clamps, or
ii). in a drum (internal device), in which case the plate precursors are held in place with compressed air or with clamps, which may be magnetic.
When a positive-working lithographic printing plate precursor is imaged on a platesetter employing clamps, the clamping device prevents the successful exposure of the coating immediately under the clamp. After development, this unexposed area of coating accepts ink. Unless this section of coating is removed manually (a time-consuming process), it will cause an unwanted image on the press. The problem is particularly troublesome for web presses, where ink is wasted and unwanted inked image areas can transfer to the back of paper stocks.
In the case of a negative-working printing plate precursor, the unexposed areas are not ink receptive following development, so the above problem is absent. There is, however, often need in making a printing plate to extend solid printing image borders to the maximum permissible area of a plate. In such cases, the areas covered by the clamps cannot be used and represent wasted space.
Rather than using clamps, some platesetters employ suction cups and powerful vacuums. On mounting a plate precursor on such a platesetter, however, at least one edge of the plate precursor is typically inserted into a crevice in the drum, where it is shaded from the imaging radiation. In such systems, the presence of unwanted, remaining image areas is therefore still not avoided. Thus there remains a need for ways of either utilizing the areas covered by the clamps or avoiding the time-consuming step of removing such unwanted image areas after plate development.